This invention relates to the combustion of sulphur-bearing fuels and more particularly to the capture and retention in solid form of sulphur and optionally sodium and chlorine or other undesirable compounds during the combustion of these fuels.
Sulphur is desirably captured and retained in a solid form during combustion to lower the amount of air pollution created by the combustion. It is desirable to capture and retain sodium and chlorine because these normally vaporize or gasify during combustion and subsequently condense on boiler heat transfer surfaces, causing slagging and fouling. Many otherwise attractive high sodium content and/or high chlorine coals are little used and are of low cost for this reason.
U.S. Pat. No. 4,523,532 issued June 18, 1985 (Moriarty et al) and U.S. Pat. No. 4,517,165 issued May 14, 1985 (Moriarty), the contents of both of which are incorporated herein by reference, disclose processes for combusting sulphur-bearing fuels. The processes disclosed in these patents have been extensively tested in two experimental combustion devices called low NO.sub.x /SO.sub.x burners. These were fired primarily with coal fuels but with a high sulphur residual oil as well. In these processes, the fuel is first combusted in a first stage, in the presence of solid sulphur binding and retaining compounds, under reducing conditions and at temperatures at which conventional thermodynamics predicts sulphur will be captured in a solid form by the binding material. The fuel is then further combusted in a subsequent stage under somewhat less reducing conditions and at temperatures higher than the fusion temperature of the binding and retaining materials. The combustion conditions in this subsequent stage are such that conventional thermodynamics predicts complete loss of the captured sulphur (i.e., oxidation to gaseous sulphur forms).
Capture of fuel-sulphur in the solid form during combustion through the use of solid binding materials is well known in the art. For example, U.S. Pat. No. 4,555,392 issued Nov. 26, 1985 (Steinberg) discloses the use of Portland cement as a sulphur-capturing material. Also, combustion conditions and binding materials for optimum sulfur capture are disclosed in the Moriarty patents, incorporated herein. However, the retention of the sulphur in a solid form through subsequent stages is not generally addressed in the prior art.